- Y (also y) ► NOUN (pl. Ys or Y's) 1) the twenty-fifth letter of the alphabet. 2) referring to an unknown or unspecified person or thing (coming second after ‘x’). 3) (usu. y) the second unknown quantity in an algebraic expression. 4) referring to the secondary or vertical axis in a system of coordinates.
English terms dictionary. 2015.